Fritz Wohlenberg GmbH (FriWo) in Hanover, with 34 employees and a turnover of around three million euros, specialises in small series production of technical rubber goods. Up to thirty different products are manufactured every day. One of the special features of the company is that all rubber compounds are made in-house. In particular, the press department with six vulcanisation systems and one injection moulding machine has developed into a growth driver in recent years. The consistent modernisation that was started in the mid-1990s has contributed to this. The highlight was the purchase of a new frame press in 2007, equipped with magnetic clamping technology from Hilma-Römheld GmbH, which enables up to twenty mould changes a day to be carried out in the shortest possible time.
The trigger for the last modernisation step was the order in 2003 from a manufacturer of testing equipment for engine blocks that regularly required rubber seals within a very short time. “In some cases, the courier who brought the moulds to the site waited for the finished products,” recalls plant manager Martin Mehlau. The seals were vulcanised under high pressure. Since the moulds with numerous different geometries had to be screwed onto the clamping surfaces of the presses at that time, the time pressure was enormous. The removal and installation process took around thirty minutes, and it took up to half an hour for the moulds to heat up and reach the right temperature for production. “At that time, it became clear to us that as a contract manufacturer specialising in individual customer solutions, we would only have a chance if we retrofit here”, says the FriWo plant manager, summarising the company's assessment.
Mehlau also appreciates the quality improvement: “Since the pressing forces are higher than on our old machines and are evenly transferred to the mould via the magnetic clamping plates, extremely homogeneous rubber products are produced. This has significantly reduced the number of rejects.”
Generally speaking, magnetic clamping technology can be used to clamp a wide variety of moulds quickly, precisely and easily. The magnetic clamping plates are suitable for both horizontal and vertical use. As soon as the mould rests and is positioned, an electrical power supply lasting two to three seconds magnetises the mould on the magnetic clamping plate. During production, current-independent permanent magnets provide the necessary force to hold even mould halves weighing several tons deformation-free, accurately positioned, parallel and reliably, thus achieving high precision in production and at the same time keeping mould wear low. Since the magnetic field only penetrates a few millimetres into the mould, it does not influence production. Even in the event of a power failure, the moulds are held securely on the clamping plates thanks to the electro-permanent magnet technology; a renewed power supply is only necessary for demagnetisation and release.
The FriWo presses vulcanise at a pressure of approx. 200 bar and temperatures between 150°C and 180°C. After the first pressing, the mould opens again completely, cold rubber is additionally refilled and the mould is closed again for about one second. The press briefly ventilates to allow the last air inclusions to escape. To do this, the plates open a gap wide. The press then closes completely. Five to six minutes later, the article is vulcanised and can be finished. “Only thanks to magnetic clamping technology, we can offer small orders like these competitively and execute them profitably”, Mehlau is pleased to report.
Wohlenberg GmbH stores around two thousand different press moulds of all geometric shapes and sizes for recurring orders, plus around twelve thousand extrusion dies. The production quantities vary greatly, says Mehlau: “In the case of custom-made products, we sometimes produce only a few copies from one mould, of other articles it is several hundred at a time.” Profile cords, hose rings and cylindrical buffers, profile cord rings, moulded articles, coupling packages belong to the standard repertoire. As a craftsman's business, Fritz Wohlenberg GmbH can also fulfil special requests such as seals for vintage cars or requests from an Arab sheikdom whose princes needed special rubber parts to convert their jeeps for desert hunting.
As varied as exotic small orders are, standard products determine everyday production. They are mainly ordered by technical traders who supply them to the chemical and petroleum industries, mechanical engineering and the food industry. But here, too, the buyers rely on the handcrafted qualities of the producer from Hanover. Wohlenberg excels above all in its development and mixing department, a service that according to Mehlau only a few companies of comparable size in the industry offer today.
According to the customer's specifications, the respective regulations and standards, the technicians at FriWo compose the recipe for the required elastomer from up to twenty different components in the desired hardness, heat resistance, elasticity, extensibility and resistance to weather, petrol, oils, alkalis and acids.
About 15 tons of natural rubber, mainly from Vietnam and Malaysia, and nearly twice the quantity of synthetic rubber are processed annually. Before further processing, our quality assurance department checks every batch of a mixture that is produced for an order. Only when the processing material fully meets all requirements, it may be used for production.
For the company, founded in 1891, the purchase of two new rubber presses was a tremendous financial feat. Nevertheless, managing director Theodor Knäbe saw the necessity and decided to purchase the presses. Due to decades of good cooperation, FriWo chose vulcanising presses from Rucks Maschinenfabrik in Glauchau, which were equipped with magnetic clamping technology from Hilma-Römheld GmbH. “Hilma was already known to us and Ruck also recommended this supplier”, recalls Mehlau.
The supplier from Siegerland is one of the leading experts for die and mould clamping and effective die or mould change in mechanical engineering, forming technology and the plastics and rubber industry. Hilma develops and produces individual clamping systems and standard fixtures for production exclusively at the Hilchenbach site. Magnetic clamping technology plays an important role. The company sells its products worldwide and is represented by the Römheld Group with sales and service companies in 39 countries on all continents.
In 2007, two vulcanising presses, each with one pair of Hilma magnetic clamping plates, were delivered to FriWo. With clamping surfaces of 600 x 600 millimetres, they are large enough to vulcanise moulds of the same height in a single process. They are networked with the three existing frame presses and the injection moulding machine acquired in 2002 and can be controlled centrally. The control programs for each mould are administered from a computer, if the mould is inserted, only the corresponding data must be loaded. A significant advantage, says Mehlau: “This ensures that recurring orders can be reproduced to one hundred percent at any time without any problems.”
With the new clamping technology, the Wohlenberg employees appreciate not only the simple repeatability of orders but also the enormous time savings when changing moulds. The plant manager adds: “If today we have to quickly produce five or six different small series in a row, this is only possible thanks to the fast magnetic clamping technology.” The time required for clamping and releasing a mould has been reduced by half an hour to five minutes. With six tool changes, the company not only saves three hours of set-up time a day, but thanks to the new clamping technology is also able to carry out specific jobs with tight deadlines.
“We were very satisfied with Hilma's service and customer support right from the start, and reliability is particularly evident when problems arise“, Mehlau sums up. Today the magnetic clamping technology works trouble-free, the employees are very pleased and can no longer imagine their everyday life without it.